Briggs Compression Question

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  #1  
Old 12-16-06, 12:15 PM
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Question Briggs Compression Question

Have a B&S model 190403 8hp mounted on a snowblower.
Was having a leaky carburetor problem (2 piece flo jet) - have gathered info on that issue but noticed quite a bit of blow-by thru carb.when starting engine.
Don't have a compression tester but removed head- cleaned carbon buildup.
Noticed on compression stroke intake valve is not tightly seated till the last 1/2" of piston travel. So I then checked intake valve clearance (.005 - .007"). It was
closer to .005" (not dramatically off). Took valve out , set piston to 1/4" past TDC on power stroke (according to some treads on this L series engine this is the proper way to set clearance - also without spring in place). Performed a slight amount of stem grinding, lapped the valve.
Reinstalled valve, checked valve clearance, rotated engine to compression stroke. Valve still doesn't seat securely till almost the entire piston travel is complete.
Shouldn't the intake be completely sealed shortly after compression stroke begins? Can anyone shed any light on this ??

Thanks,
 
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  #2  
Old 12-16-06, 03:21 PM
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I don't find that number in the Briggs single cyclinder OHV manual, so i assume that this is an L head engine (valves in block and lifted directly by the camshaft). If you had the sump cover off to remove and grind the stem You may have let the cam gear disengage and have the engine out of valve timing. If this is the case, you will have to take the sump cover back off and line up the two dots on the crank and camshaft gears.
If any thing that I have submitted sounds like your engine, fine. If not describe the engine to me more clearly and I sure will try to help you. Tom
 
  #3  
Old 12-16-06, 04:11 PM
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Yes the intake valve should close somewhere around the beginning of the compression stroke. When an engine backfires through the carb, that is a sign of the intake vave burned (unusual), set up to tight and not fully closing, sticking in the guide or the cam to crank timing has changed. If you had not said that the valve was not closing until I/2" before TDC, then I would tell you to pull the magneto wheel and see if the key was bent or sheared, but that does not seem to be the case here. You could sell out, move south where we are and for the most part don't know what snowblowers are. Ha!!
 
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Old 12-16-06, 05:27 PM
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abbey,Take the valve back out and check the valve stem for,and remove all burs from the stem.And clean the valve guide.The valve has to slide up & down EZ before you check the valve clearnce,check to see if valve is bent.also where you grind on the valve stem to get the proper clearance,it has to be square.If not you'll get a falce reading.
 
  #5  
Old 12-16-06, 06:47 PM
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gardener321,
Thanks for your input.
You are correct. It is an 'L' series engine. I discovered this situation before I
did anything (i.e., removed the Intake Valve).
Of course first I checked the valve clearance also.
That would eliminate me causing a cam timing issue.
I did wonder about cam timing though as a pre-existing condition but the engine
has always run fine... maybe a little hard to start from time to time (probably as a result of this valve issue) but fine otherwise. Again, what led me to discover this was while I was working on the carb issue.
To look at the Intake Valve you would think it's seated - until you put your thumb (or lapping tool) on it while turning the crank and spinning the valve.
At that point you realize it's not firmly seating until the last little bit of Pistion travel. My other thought was: did someone install an incorrect llength Intake Valve at one time?
I bought it used (practally new at the time) and have owned it for 20+ yrs., never stopped the engine with a foreign object and haven't touched the Valves so I don't think that's the case...
?????????????????
 
  #6  
Old 12-16-06, 06:55 PM
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Thanks repair_guy,
Did that initially and several times thereafter.
Valve was clean (face,stem,base -no burrs). Moves freely in guide.
Tappet is clean also.
Could the Tappet be getting hung up?
Maybe I'll take the Valve off again and see if I can feel the Tappet movement...
 
  #7  
Old 12-16-06, 07:58 PM
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It's possible there may be wear in bearing surfaces in the block or sump that is letting the camshaft move or wobble some while it rotates. This would affect the valve operation, impossible to tell without taking the engine down.

Just a thought.....

Best of Luck....
 
  #8  
Old 12-17-06, 12:37 PM
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An 8hp Lhead engine is certainly not a complex piece of equipment. As 30yrtech has stated, along with the age of the unit, it may be time to pull the PTO cover and see whats going on. I do not ever remember running in to one of these engines that has jumped time because of jumping gear teeth. but it does sound to me that you are having a problem that would relate to that type of situation. I don't know, so am asking if anyone knows whether the cam gear and cam are one piece or two separate pieces because in my reasoning, in any other type of situation, this would be a timing problem. Abbey says nothing is sticking and declares that the piston is half an inch from TDC before the valve closes. If the cam and gear are two pieces and the two parts were heat shrunk fitted or even keyed, might not this have happened? I can almost assure you that if the valve does seat and seal and does not do so until half an inch before TDC, a compression gage would not indicate enough press to do anything put pop back through the carb. Furthermore if the timing is right and the valve is sealing, it will more than likely not spit back thru the carb anyway. I am not coming on as an expert here just a confused guy who would pull the cover and take a look at the innards. Tom
 
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Old 12-17-06, 05:42 PM
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The cam gear in all lawnmower engines are made in one piece.I have run into problems to where the whole cam shaft was broken in half right below the gear,but nighter valae would work.The crankshaft end barnings being wore so badly as to let the crankshaft woble and let the cam slip a tooth or two as 30 year has suggested sounds possiable,But it also seams to me that that condention would tare up the oil seals and cause the engine to leak motor oil.I tend to lean tawords the theory of a bent valve stem,or head.or mayby a weak valve spring.

Only times I've run into a valve not seating has been caused by a mismatched valve seat to valve face serfice.Or too long of a valve steam.
So when you do find the problem,post it....let us know.Or shoulh I say "let me know".
 
  #10  
Old 12-17-06, 05:53 PM
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It is strange... more so since it was running ok ....even with the carb problem...

I did closely look at the Exhaust Valve operation today (assuming if there's a timing issue it should effect the Exhaust Valve also).
Exhaust Valve is seated thru intake,compression, and most of the power stroke.
It begins to become unseated with about a 1/2" of piston travel left (BDC) during Power stroke.
Sure looks like something occurred within the Block.....
I'll let you know.
Thanks,
 
  #11  
Old 12-17-06, 05:58 PM
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I don't think nor did I suggest that the cam slipped a tooth at all. I am simply speculating that if the cam is loose in the support bearings that it can move enough to affect the valve movement.

A loose or worn cam bearing will not leak any oil as they do not have any seals!

A bent valve stem would most likely stick all the time and not at a particular point in the compression stroke, and as abbey stated the valve moves freely and has no burrs.

Since it appears to be a timing issue, worn bearing surfaces for the camshaft is just a thought, I have seen this before but only a couple of times, this can cause erratic valve operation when the camshaft is installed correctly and properly timed.
 
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Old 12-17-06, 06:39 PM
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I mis-understud which barning you were speaking of.My bad,sorry.

After seeing it all in my mind,the only thang that makes any since would be a wore out or cracked upper cam barning or a bent cam.The bottom barning in the sump and the teeth on the crank would hold the cam from wobbleing,(there so close together) which would not effect the ex.valve.Only the intake.

By george watson,I beleive you've got it.


I've seen this proiabley 1000 times myself,Just forgot.
 
  #13  
Old 12-17-06, 08:43 PM
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Sounds that we all agree, more or less, on one thing. Its pretty simple to pop the pto cover off and take a look see. I still say mechanical timing problem. However I sure want to know what you find Abbey please don't leave us out. I may have to sit in the corner with the long pointed cap on. Ha Hey learning something new to me is a pleasure not and insult. lol
As far as an upper cam bearing, unless I read the mod codes wrong, this is a horizontal crank engine with a 120/240vac elec starter.
 
  #14  
Old 12-18-06, 05:37 AM
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Smile

Yes Gardner321 it is an Horizontal engine with an electric starter.
I'll let all know what I find hopefully in the next week or so.
Watch ... we'll get dumped on with the white stuff in the mean-time...(:>)
 
  #15  
Old 12-18-06, 05:53 AM
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If its a wear problem it might show up if the engine was rotated in reverse and check the valves to see if they operate (Open and Close) at the same time as when rotating it normally.
 
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Old 12-18-06, 10:21 AM
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Originally Posted by abbey View Post
It is strange... more so since it was running ok ....even with the carb problem...

I did closely look at the Exhaust Valve operation today (assuming if there's a timing issue it should effect the Exhaust Valve also).
Exhaust Valve is seated thru intake,compression, and most of the power stroke.
It begins to become unseated with about a 1/2" of piston travel left (BDC) during Power stroke.
Sure looks like something occurred within the Block.....
I'll let you know.
Thanks,
If I understand you correctly, the ex valve is opening when the piston is ALL BUT i/2" of the way down on the power stroke. This is normal in combustion engines. Since the piston travel is much more between tdc and 90 deg past that point, engineers open the exhaust vlave at approx 165 deg of crank travel. The piston at that time has used up the force of combustion, and this allows the exhaust gas under slight press to start purging the ex gas as it rises on the exhaust stroke. So that part of it means nothing. lol
 
  #17  
Old 12-19-06, 12:29 AM
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Sounds like the exhaust valve operation is normal, as gardener321 indicated. The intake valve operation is questionable though. I would agree that there may be a problem with the cam bearings/bosses. I've seen the bosses crack on the block side, usually due to a thrown rod. It's often overlooked or ignored when the engine is repaired, and begins to cause problems later on. Has this engine ever broken a rod that you know of? I would expect this kind of problem to cause the valve to stay closed longer, rather than open too long.
 
  #18  
Old 12-28-06, 06:27 PM
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I hope to look into this further this weekend.
It did put the head back on and checked compression.
I'm surprised it ran at all (about 60).
Also did leak down test - Intake Valve is definately leaking.
I agree Cheese - I would think that if components were worn the Valve would stay closed longer (as opposed to staying open)...
With the little grinding and lapping I've done so far, I'm right at the max lash tolerance (.007"). Hate to take any more off....
 
  #19  
Old 01-05-07, 06:07 PM
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I guess what I'm seeing (related to the Intake Valve) is normal.
The following is quoted from Oct. 81 Service Manual:

"The intake lobe on the cam gear is ground with a small ramp which holds the intake valve open 1/100th of an inch for a tiny fraction of the compression stroke. At slow starting speed the interval of the time that the valve is open is relatiively long and therefore enough air escapes to noticeably reduce the compression. However,at operating speeds the interval of time is so short that there is practically no escape and therefore horsepower is unimpaired. Actually at 3600 rpm the valve is opened for a mere 1/200 of a second.
In all other respects the valves operate as in any other four stroke engine.
The force required to start is reduced by 50% with Easy Spin and would be noticed most by a person who has difficulity starting an ordinary engine.
When testing the compression of an Easy Spin engine one must spin the flywheel
backward, in the opposite direction to normal rotation. This will bring the compression stroke on the opposite side of the cam lobe and allow you to feel the compression"
 
  #20  
Old 01-05-07, 11:55 PM
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I was wondering if you were describing the easy spin feature, but I don't think I realized it held the valve open that long during visual inspection. I guess I never noticed. To test compression the Briggs way, remove the plug wire, and give the engine a spin backwards. If the engine has enough compression to bounce the reverse motion forward a bit when it hits the compression stroke, it's generally good enough.
 
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